Email bounce-did not report a specific reason #5.0.0 smtp; 550 5.0.0 Access denied

i am really perplexed. We have no problem sending and receiving email to anyone except one client from Australia. The sysadmin claiims emails to our network is being bounced back within 30 seconds stating the message did not reach some or all of the intended recipients. "The email system was unable to deliver the message but did not report a specific reason. Check the address and try again. If it still fails, contact your system administrator.  <cname.domain.com #5.0.0 smtp; 550 5.0.0 Access denied>" I know that this error is a very generic SMTP error just not sure where it is emanating from.
oppositelockAsked:
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ikm7176Sr. IT ManagerCommented:
check out these Possible causes for this

- There is no route for the given address space; for example, an SMTP connector is configured, but this address does not match.
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 DNS returned an authoritative host that was not found for the domain.
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 The routing group does not have a connector defined; mail from one server in one routing group does not have a route to another routing group.
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 An SMTP error occurred.

workaround:

On one or more SMTP connectors, add an asterisk (*) value as the SMTP address space; verify that DNS is working; ensure that routing groups have connectors connecting them. If necessary, apply the latest Exchange service pack (SP3) to isolate the issue.
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BembiCEOCommented:
Within your virtual smtp server, enable the smtp log with all options and wait for the next bounced mail. Then search the log for the senders address to see, what is the reason for that. There may several reasons, beginning from spam filters, blocking lists, header problems if sent from exotic mail clients, firewall restrictions and of couse of misconfigurations. As it affects only one single sender, I would assume, that is not really your problem. Nevertheless, the log protocol may show you the answer.

If you have installed the IIS lock down tool, there may be some SMTP flags, which are filtered out. But the access denied message points me either to a block entry somewhere or to a spam filter.
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